Everytime i decellerate stepping on the brakes and the speed gets down to 30-35mph the rpm will make a sudden drop to 800, then go up 1,000 then down again till it reaches idle rpm thats when the car is fully stoppped. Also i feel hesitation when accellerating, does this sound an exhaust leak or plenum vacuum leak i have AAM spacer but the idle rpm stays in place @ 750.

 

Is the idle rough? Can you hear hissing when idling (putting your ear near the plenum)?